1. Field of the Invention
The present invention relates to a display device and a method of manufacturing the same. More particularly, the present invention relates to a display device having thin film transistors formed on a display substrate thereof and a method of manufacturing the same.
2. Description of the Related Art
For example, in an organic electroluminescence (EL) device, pixels are arranged in matrix on a display substrate of the device, and each of the pixels includes an organic EL element. Light emissions by the organic EL elements themselves display an image.
Further, a plurality of pixels arranged in a row direction, for example, are selected in sequence in a column direction. In synchronization with the timing of the selection, a signal (video signal) is supplied through a signal line (drain line) formed in common to a plurality of pixels arranged in the column direction, and the respective pixels are driven by current corresponding to the signal flowing into the organic EL elements.
Therefore, each of the pixels includes at least a switching element for selecting the plurality of pixels arranged in the row direction and a current control element for allowing current corresponding to the video signal to flow into the organic EL element. Both of the switching element and the current control element are formed by a thin film transistor having a metal insulator semiconductor (MIS) structure.
As a structure of such a thin film transistor, there is known one disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-167051, and a sectional view thereof is illustrated in FIG. 2. It is to be noted that the view of FIG. 2 corresponds to the view of FIG. 1B illustrating an embodiment of the present invention, and hence description of FIG. 2 is limited to schematic description. The detailed structure of FIG. 2 is to be referenced by the description related to FIG. 1B.
In FIG. 2, a thin film transistor (TFT) has a so-called bottom gate structure. Specifically, on a gate insulating film 12 covering a gate electrode 11, a laminate (semiconductor laminated film 16) formed by laminating in sequence a polycrystalline semiconductor film 13, an amorphous semiconductor film 14, and an amorphous semiconductor film heavily doped with impurities (heavily doped amorphous semiconductor film) 15 is formed on the gate insulating film 12 in an island-like manner. A drain electrode 17 and a source electrode 18 are formed on top of the semiconductor laminated film 16. In this case, the heavily doped amorphous semiconductor film 15 between the drain electrode 17 and the source electrode 18 is removed to such an extent that the amorphous semiconductor film 14 is exposed so as to function as an ohmic contact layer. The drain electrode 17 and the source electrode 18 each are formed so as to extend to outer peripheral portions of the semiconductor laminated film 16 along side-wall surfaces of the island-like semiconductor laminated film 16.
It is to be noted that a drain electrode and a source electrode of the thin film transistor (TFT) operate interchangingly depending on a bias application state. However, in this specification, for the sake of simplicity of the description, a left electrode in the drawing is referred to as a drain electrode while a right electrode in the drawing is referred to as a source electrode. There may be a case where one of the drain electrode and the source electrode is referred to as a first electrode, whereas another one is referred to as a second electrode.
Further, T. Kaitoh et al., “SELAX Technology for Poly-Si TFTs Integrated with Amorphous-Si TFTs”, pp. 481, International Display Workshop 2007 discloses a structure of a thin film transistor (TFT) as illustrated in FIG. 3. The view of FIG. 3 corresponds to the view of FIG. 2. In FIG. 3, the structure is different from that illustrated in FIG. 2 in that the heavily doped amorphous semiconductor film 15 is not only formed on a surface of the amorphous semiconductor film 14 but also extends to outer peripheral portions of the semiconductor laminated film along side-wall surfaces of an island-like semiconductor laminated film (amorphous semiconductor film 14 and the polycrystalline semiconductor film 13), and in that the drain electrode 17 and the source electrode 18 are formed superimposingly on the heavily doped amorphous semiconductor film 15.